Demonstration that the loss of contrast against a hot CMB is not much alleviated for single lines of sight by using the HCN(2−1) line. For the parameters discussed in Section 2.2 the HCN(2−1) line is affected almost identically to the HCN(1−0) line. Results shown here for the hot and cold limits of 10² K and ﹩{10}^{0.5}﹩ K on the earlier models for ﹩{\bar{n}}_{{{\rm{H}}}_{2}}=1\times {10}^{4}﹩ cm⁻³ (left) and ﹩{\bar{n}}_{{{\rm{H}}}_{2}}=1\times {10}^{5}﹩ cm⁻³ (right).